This invention relates to an improved graphite resistance heater for use in a high-pressure high-temperature reaction vessel. More particularly, this invention relates to a graphite heater having at least a partially metallized surface.
Synthetic diamond, polycrystalline diamond, cubic boron nitride, and other super abrasives that are used in a variety of tools and wear resistant products are manufactured at super high pressures and temperatures in specially designed reaction vessels. The reaction vessels are loaded into high-pressure high-temperature presses capable of exerting pressures on the vessel of between 40 to 80 kilobars. In the course of this process a high electrical current from an external source is passed through a graphite tube heating mechanism within the vessel, known in the art as a heater, raising the temperature within the reaction vessel to within the zone where diamond is thermodynamically stable. Presses of this nature are well known in the art as belt, cubic, and prismatic presses, or variations thereof.
The graphite heater is positioned concentrically inside the reaction vessel and subjected to ultra high pressures and temperatures. A payload consisting of diamond, polycrystalline diamond, cubic boron nitride, or other super material is loaded inside the graphite heater where temperatures will be the highest. The graphite heater has thin walls in order to present a resistance path for the high electrical current, thereby creating heat inside the vessel. As the pressure is increased on the reaction vessel, the vessel and its components consolidate, often causing the thin, fragile walls of the heater to fracture. Contaminants from the surrounding components are forced into these fractures, interrupting the current path through the heater. Fractures in the heater and contaminants cause hot and cold spots along the surface of the heater, and are also linked to failure of the heater altogether. Uneven heating of the payload is a cause of low product yields from the press, and heater failure has been attributed to anvil breakage and catastrophic failures during the high-pressure high-temperature press cycle.
Another deficiency with the graphite heater arises when the reaction vessel is fitted with a thermocouple. The wires of the thermocouple are positioned in contact with the graphite heater and rely on the graphite tube for an electrical connection. But at the temperatures reached in the high-pressure high-temperature process, the materials of the wire react with the graphite causing erratic and inconsistent readings. Again poor temperature control within the reaction vessel is another cause of low product yields.
Therefore, it is desirable to strengthen the heater in order to increase product yields and to reduce anvil breakage and catastrophic failure during the operation of high-pressure high-temperature press apparatus. This invention overcomes the propensity of the graphite heater to fracture during the high-pressure high-temperature process. This invention provides a heater having a metallized surface that constrains the graphite tube during consolidation under pressure, imparts toughness to the graphite, shields the graphite tube from outside contamination, and provides an improved electrical contact when a thermocouple is fitted into the reaction vessel. Additionally, it accommodates higher current that will permit faster heating of the payload.